...and I'd like a pony... and I'd like to have that fact mean I'll be getting one soon.
But that ain't the way the real world works... nor is specifying a version always based on any real-world reality.
Do you have any method for demonstrating 'truthfulness' in your statement (below) that you "can't ask them (users) to upgrade" from an obsolete version? If you're providing the programming expertise, you clearly have standing to -- at a minimum -- suggest they install a version that has overcome certain known bugs or added abilities. They don't have to delete 5.8 which, likely, is used for system functions; you'll find many threads here about how to ADD a current Perl version to a system without borking the system-Perl.
BTW and FWIW, the fact that RH says it will support 5.8 until 2017 for RHEL5 does NOT mean RH will cure the bugs (nor the shortcomings) of 5.8 compared to 5.10, 5.12, etc.
Sometime, it takes a 'clue by four' to get a mule to do the right thing.